Verifying compliance with the terms of a probation, parole, or a release is a difficult and onerous task for the authorities charged with that responsibility. In the past, periodic meetings and telephone conversations were usually the only mechanisms which could be utilized to locate and supervise individuals convicted of criminal acts who were not institutionally confined or incarcerated. However, recent technical innovations coupled with new criminal justice procedures have introduced alternative sentencing mechanisms. In particular, home confinement systems have been recently developed that permit a convicted individual to be restricted to his home, or a designated locality, to serve out a term of sentencing. Such systems have also been used to monitor the whereabouts of parolees and probatees whose freedom to travel has been restricted.
However, most of the presently available home confinement systems are based on radio frequency (RF) communications. In an exemplary RF system, an individual under surveillance carries an anklet or bracelet radio transmitter which transmits continuously to a centrally-located radio receiving station which monitors the presence of the individual at a designated locality. The major drawback with such a system is that it is subject to outside interference which can be caused by many different sources, for example, house obstructions; electrical fields produced by electric motors, fluorescent lights, and the like: environmental conditions: and external radio signals. Further, if power transmission is raised to overcome an interference, an RF system may cause radio interference with third party devices, such as televisions or telephones, and possibly violate federal communication regulations. Second, RF systems are passive monitoring systems which do not provide a positive identification of an individual under surveillance but instead rely on the security of the anklet/bracelet connection. In addition, presently available RF systems lack the capability to immediately retest the presence of an individual in the event the system cannot detect the individual's presence. Therefore, in the case of a system failure or interference, an RF system may set off a false alarm.
Active monitoring systems that carry out voice verification tests over standard telephone lines have also been developed. Although these systems may not experience the aforementioned problems of RF systems, they nonetheless are subject to various technical problems. For example, the standard telephone handset is designed to transmit only the necessary frequencies and amplitudes associated with intelligible voice transmission and not all the characteristics of speech needed for voice verification. In addition, the standard telephone handset inherently has a limited dynamic range that will introduce severe harmonic distortion outside its dynamic range and, thus, can cause false voice recognition. Further, ordinary telephone lines used in such systems will introduce additional distortion because of their limited dynamic range and noise that regularly appears on telephone lines (e.g., background hiss, ringing, cross talk, and echoes). Devices and techniques used to overcome these limitations can be costly to incorporate into an active monitoring system and may degrade a system's performance.